What a floor slab looks like inside
In most Spanish concrete blocks of flats the slab spans one way: long precast pieces, the joists (viguetas), run from support to support; hollow clay or concrete blocks fill the gaps between them, and a layer of reinforced concrete is poured on top. Spain's Structural Code (Royal Decree 470/2021) describes exactly that: joists or ribs, infill blocks, the concrete topping poured on site and the reinforcement together form the load-bearing floor.
There are other types: solid concrete slabs, waffle slabs with ribs in two directions, steel-deck and concrete floors and, in older buildings, timber floors. As a guide to their weight, the CTE gives about 3 kN/m² for a one-way slab spanning up to 5 metres with a total depth under 28 centimetres.
Why it matters if you are buying
The slab is part of the structure, a common element of the building. Spain's Horizontal Property Act makes the works needed to keep the building in good repair compulsory without a prior vote of the owners' meeting, and each owner pays according to their ownership quota. If the floors need strengthening, that special levy (derrama) will be yours too.
Cracks running along the joists, rust stains on the ceiling, pieces of concrete coming away or a sagging floor are signs a professional should see. Knowing what the slab is made of and when it was built also tells you which problems are typical: in concrete joists from a certain period, high-alumina cement; in timber floors, damp or wood-boring insects.
What the HOUSINGFAX report tells you
Check 01, «Building condition assessment», sets out what the registered building inspection says in the Valencian Community, the Basque Country and Madrid: date, result and, where the register publishes them, the defects by element, such as the structure, with their deadline. Checks 02, «Risks by year of construction», and 03, «Aluminosis», explain what usually happens in the floors of a building of that age.
That is 22 checks with an official source and a date, in one report. Look through the sample report and start with the free one: check your home for free.
What no record can tell you
The real condition of a floor slab can only be known by looking at it, often by lifting a suspended ceiling or going into a storage room or garage. The report shows whether the records contain warning signs and tells you what to look into, but it does not replace a technical inspection of the building.
Short answers
What is the difference between a floor slab and a joist?
The slab is the whole assembly that forms the floor of a storey; the joist is one of its parts, the piece that runs from support to support and holds up the rest.
What is a one-way slab?
One that works in a single direction: the joists or ribs run parallel between two supports and infill blocks fill the space between them. It is the most common type in concrete blocks of flats.
Who pays to repair a floor slab?
As part of the structure, the community of owners pays, and each owner contributes according to their ownership quota unless the title deed or the bylaws set a different split.
How can I tell whether the floors of a flat are sound?
Look at ceilings and walls during the viewing, ask the community for the latest building inspection and read what the report says. If you see cracks, rust or a sagging floor, have a professional look before you sign.
Official sources you can consult
The links let you review the primary source. Their inclusion does not widen their purpose or turn a piece of context into a diagnosis.